1. Field
The presently disclosed subject matter relates to an optical semiconductor device such as a light emitting diode (LED) and its manufacturing method.
2. Description of the Related Art
Generally, in a prior art optical semiconductor device, an AlGaInP light emitting layer lattice-matching with GaAs and a GaInP current spreading layer not lattice-matching with GaAs are sequentially and epitaxially grown on a semiconductor growing GaAs substrate. Then, a reflective mirror is deposited thereon by a chemical vapor deposition (CVD) process or a sputtering process, to obtain a semiconductor laminated body. Then, the semiconductor laminated body is bonded to a support body. Finally, the GaAs substrate for absorbing a visible light component of light emitted from the AlGaInP light emitting layer is wholly removed (see: JP2006-86208A and JP2008-98336A). Thus, since the visible light absorbing GaAs substrate is wholly removed, light radiated from the AlGaInP light emitting layer to the reflective mirror is totally reflected at the reflective mirror to reach a light extracting face opposing the reflective mirror, so that a part of the totally-reflected light is extracted therefrom to the exterior, which would improve the light extracting efficiency.
Another prior art optical semiconductor device uses a highly-reflective metal layer made of Ag, Au or Al as the above-mentioned reflective mirror (see: JP2000-349349A). In this case, the reflectivity of the metal layer has no dependence upon the incident angle of light from the AlGaInP light emitting semiconductor layer to the reflective mirror. However, since the reflectivity of the above-mentioned metal layer is about as large as 95%, the light from the AlGaInP light emitting semiconductor layer cannot be completely reflected at the reflective mirror, so that the light extracting efficiency is not so high.
A further other optical semiconductor device uses a combination of a silicon oxide (SiO2) layer and a metal layer as the above-mentioned reflective mirror (see: JP2006-165257A). In this case, the reflectivity of the silicon oxide layer is about 100% for light incident thereto having an incident angle larger than the critical angle. This will be explained later in detail.
In the above-described further other prior art optical semiconductor device, however, the reflectivity drops in the proximity of the critical angle, so that the reflectivity is about as small as 75%. Also, the reflectivity is about as small as 96% in the incident angle from 0° to 15° smaller than the critical angle. As a result, the light extracting efficiency is still low. Further, the margin of the optimum thickness of the silicon oxide layer is so small that the manufacturing cost would be increased.
Note that JPHei7-193275A, JPHei8-116088A, JPHei8-316526A, JPHei10-125953A and JPHei10-341034A disclose an optical semiconductor device which is constructed by a light emitting semiconductor layer epitaxially-grown on a semiconductor growing substrate while an air gap is provided between the light emitting semiconductor layer and the semiconductor growing substrate. In this case, the air gap serves as a reflective mirror whose reflectivity is larger than that of silicon oxide (SiO2). However, since the semiconductor growing substrate is not removed and the reflective mirror includes no reflective electrode layer for reflecting light penetrated through the air gap, the effective reflectivity of the reflective mirror regarding light whose incident angle is smaller than the critical angle is not so large, so that it is impossible to enhance the light extracting efficiency.